The present invention relates to an optical bus transmission method in which Manchester-biphase digital electric signals are converted into optical signals and transmitted over an optical bus. The invention also pertains to a transmitting-side encoder and a receiving-side decoder therefor.
There is known a DATAC (Digital Automous Terminal Access Communication) system as LAN (Local Area Network) introduced for digital data communication between various pieces of electronic equipment loaded on aircraft (for example, see the magazine "NEC Giho," Vol. 39, No. 12, 1986, pp 160-167). The DATAC system transmits data as a digital signal of the Manchester biphase. The digital signal of the Manchester biphase is transmitted as two pulse trains of TXO and TXN signals, accordingly two buses or transmission lines are needed for transmitting them separately of each other.
It is deemed possible to employ an optical bus in the DATAC system, but two way transmission is impossible with a single optical bus because of the use of an optical coupler. In this instance, a total of four optical transmission lines are needed, two for optical transmission of the TXO signal in two directions and two for optical transmission of the TXN signal in two directions--this inevitably enlarges the scale of the system.
Moreover, when many terminals are connected to the optical bus, the total transmission loss by the optical couplers is relatively large, and consequently, an optical signal received from a near terminal and an optical signal received from a distant terminal greatly differ in level. If provision is made for receiving even an optical signal of a low level, there is a possibility that the receiver becomes saturated in the case of receiving an optical signal of a high level and that seriously distorts its waveform, resulting in a failure to detect a correct pulse.
Further, in order for each terminal to receive a signal at a sufficient level, it is necessary to provide an optical signal of large optical power on the optical bus, but if pulses of a conventional electric signal are used intact, then a signal of large optical power cannot be output.
When a plurality of terminals simultaneously output signals to the bus, it is impossible to obtain correct data. On this account, each terminal starts transmission after detecting what is called a bus quiet state in which no carrier is present on the bus. When a plurality of signals are simultaneously provided onto the bus by some cause, that is, when a so-called signal collision state occurs, each terminal detects it and stops its signal transmission. With the prior art system of the type that replaces an electric signal with an optical one, when a signal of large level and a signal of small level, both relatively large in their pulse width, overlap, it is difficult to detect the collision of such two signals.